1. Field of the Invention
The present invention relates to a water decomposition apparatus and a water decomposition method that receives light to decompose water to generate hydrogen gas and oxygen gas, and particularly, to a water decomposition apparatus and a water decomposition method that includes a mechanism which shields a hydrogen generation electrode including an inorganic semiconductor film having a pn junction and a hydrogen generation catalyst at regular time intervals, and generates hydrogen gas and oxygen gas.
2. Description of the Related Art
In the related art, as one of the forms in which solar light energy is utilized as renewable energy, hydrogen production apparatuses that use a photoelectric conversion material utilized for solar batteries, and produce hydrogen gas used for fuel cells or the like, utilizing electrons and holes obtained in photoelectric conversion for an electrolysis reaction of water are suggested (for example, refer to JP2012-177160A and JP2004-197167A).
Additionally, as one of the forms in which visible light, such as solar light, is utilized, a hydrogen generation apparatus that uses a photocatalyst, and decomposes water to produce hydrogen gas, utilizing the photocatalyst for a reduction reaction of water, is suggested (for example, refer to JP2012-052184).
Both the hydrogen production apparatuses disclosed in JP2012-177160A and JP2004-197167A disclose that a photoelectric conversion part or a solar battery in which two or more pn junctions that generate an electromotive force if solar light is radiated are connected in series is provided, an electrolytic solution chamber is provided on a lower side of the photoelectric conversion part or the solar battery opposite to a light-receiving surface that receives solar light on the upper side of the photoelectric conversion part or the solar battery, the inside of the electrolytic chamber is divided by an ion-conductive partition wall or a barrier, and hydrogen gas is generated by electrolyzing water with electric power generated in the photoelectric conversion part or the solar battery through reception of solar light.
Since the hydrogen production apparatus disclosed in JP2012-177160A can further adjust the orientation of the light-receiving surface with respect to solar light, the amount of incident light to be subjected to photoelectric conversion can be increased, and hydrogen generation efficiency cannot be decreased.
Additionally, since the hydrogen production apparatus disclosed in JP2004-197167A electrolyzes water using electrode plates connected to p-type and n-type semiconductors of the solar battery as an anode and a cathode, respectively, the efficiency of conversion from solar energy to hydrogen gas can be made high.
Additionally, the hydrogen generation apparatus disclosed in JP2012-052184 has a photocatalyst obtained by doping 4 to 10 mol % of Rh in a Ti site of SrTiO3, uses a photocatalyst electrode showing p-type semiconductor properties, and can generate hydrogen under radiation of visible light, such as solar light, even under an application condition of an external bias, which is lower than a theoretical decomposition voltage of water.
Additionally, JP2012-052184 discloses that, under the application conditions of the external bias, if controlled-potential electrolysis measurement is performed while visible light is radiated intermittently (at one-minute intervals) over a long time of 25 hours, the amount of a cathode current resulting from a reduction reaction of water increases gradually together with light irradiation time and has no great change even if visible light is shielded in the middle. Additionally, JP2012-052184 discloses that, under no application conditions of the external bias, if controlled-potential electrolysis measurement is performed while visible light from a xenon lamp is radiated intermittently (at one-minute intervals) over a long time of 25 hours, the amount of the cathode current resulting from the reduction reaction of water decreases gradually together with the light irradiation time and recovery thereof is shown even if visible light is shielded in the middle, but in a case where the controlled-potential electrolysis measurement is performed while solar light from a solar simulator is radiated intermittently (at one-minute intervals), the amount of the cathode current decreases gradually after having increased gradually together with the light irradiation time, and has no great change even if visible light is shielded in the middle.